Timeline Cosmology • 2nd Millennium BCEBC Mesopotamian cosmology has a flat,circular Earth enclosed in a cosmic Ocean • 12th century BCEC Rigveda has some cosmological hymns, most notably the Nasadiya Sukta • 6th century BCE Anaximander, the first (true) cosmologist ‐ pre‐Socratic philosopher from Miletus, Ionia ‐ Nature ruled by natural laws ‐ Apeiron (boundless, infinite, indefinite), that out of which the universe originates • 5th century BCE Plato ‐ Timaeus ‐ dialogue describing the creation of the Universe, ‐ demiurg created the world on the basis of geometric forms (Platonic solids) • 4th century BCE Aristotle ‐ proposes an Earth‐centered universe in which the Earth is stationary and the cosmos, is finite in extent but infinite in time • 3rd century BCE Aristarchus of Samos ‐ proposes a heliocentric (sun‐centered) Universe, based on his conclusion/determination that the Sun is much larger than Earth ‐ further support in 2nd century BCE by Seleucus of Seleucia • 3rd century BCE Archimedes ‐ book The Sand Reckoner: diameter of cosmos ~ 2 lightyears ‐ heliocentric Universe not possible • 3rd century BCE Apollonius of Perga ‐ epicycle theory for lunar and planetary motions • 2nd century CE Ptolemaeus ‐ Almagest/Syntaxis: culmination of ancient Graeco‐Roman astronomy ‐ Earth‐centered Universe, with Sun, Moon and planets revolving on epicyclic orbits around Earth • 5th‐13th century CE Aryabhata (India) and Al‐Sijzi (Iran) propose that the Earth rotates around its axis. First empirical evidence for Earth’s rotation by Nasir al‐Din al‐Tusi. • 8th century CE Puranic Hindu cosmology, in which the Universe goes through repeated cycles of creation, destruction and rebirth, with each cycle lasting 4.32 billion years. • • 1543 Nicolaus Copernicus ‐ publishes heliocentric universe in De Revolutionibus Orbium Coelestium ‐ implicit introduction Copernican principle: Earth/Sun is not special • 1609‐1632 Galileo Galilei ‐ by means of (telescopic) observations, proves the validity of the heliocentric Universe. • 1609/1619 Johannes Kepler ‐ the 3 Kepler laws, describing the elliptical orbits of the planets around the Sun • 1687 Isaac Newton ‐ discovers Gravitational Force as agent behind cosmic motions ‐ publishes his Principia (Philosophiae Naturalis Principia Mathematica), which establishes the natural laws of motion and gravity (the latter only to be replaced by Einstein’s theory of GR) • 1755 Immanuel Kant ‐ asserts that nebulae are really galaxies separate from and outside from the Milky Way, ‐ calling these Island Universes • 1785 William Herschel ‐ proposes theory that our Sun is at or near the center of ou Galaxy (Milky Way) • 1826 Heinrich Wilhelm Olbers ‐ Olber’s paradox (why is the night sky dark ?) • 1837 Friedrich Bessel, Thomas Henderson, Otto Struve ‐ measurement parallax of a few nearby stars: the first measurement of any distances outside the Solar System. ‐ establishes the vast distances between the stars • 1848 Edgar Allan Poe ‐ first correct solution to Olber’s paradox in Eureka: A Prose Poem, an essay that also suggests the expansion of the universe • 1860 William Huggins ‐ develops astronomical spectroscopy : Orion nebula is mostly made of gas, the Andromeda nebula dominated by stars • • 1905 Albert Einstein ‐ Special Theory of Relativity ‐ space and time are not separate continua, instead they define a 4‐dim. spacetime continuum • 1915 Albert Einstein ‐ General Theory of Relativity: Einstein field equations ‐ represents an entirely new theory of gravity, in which gravity is the result of the local curvature of space, hence replacing the action‐at‐a‐distance theory of Newton. ‐ spacetime becomes a flexible dynamic medium, warped by energy density • 1917 Willem de Sitter ‐ first general relativistic cosmology, de Sitter Universe ‐ empty expanding Universe with cosmological constant • 1912 Henrietta Leavitt ‐ Cepheid variable stars period‐luminosity relation ‐ crucial step in measuring distances to other galaxies • 1920‐1921 Harlow Shapley & Heber Curtis ‐ Shapley –Curtis debate or “Great Debate”, National Academy of Science ‐ debate on the distances to spiral nebulae: are they individual galaxies like the Milky Way or are they part of the Milky Way • 1923 Edwin Hubble ‐ measures distance to few nearby spiral nebulae (Andromeda Galaxy, Triangulum galaxy, NGC 6822) ‐ distances place them far outside our Milky Way ‐ demonstrates that the spiral nebulae are galaxies outside our own Galaxy, the Milky Way ‐ In other words, the Galaxy loses its central unique position and the Universe turns out to be much, much larger • 1922 Vesto Slipher ‐ finds that spiral nebulae are systematically redshifted, ie. moving away from us • 1922 Alexander Friedmann ‐ Friedmann solution to the Einstein field equations, now known as Friedmann‐Robertson‐Walker‐Lemaitre equations ‐ solutions for a perfectly uniform space ‐ imply expansion of the space • 1927 Georges Lemaitre ‐ solutions for Einstein field equations, for a perfectly uniform space, confirming Friedmann ‐ discusses the implications, that of an expanding Universe and the creation of the Universe ‐ predicts distance‐redshift relation (later known as Hubble relation) ‐ may indeed have discovered the expansion of the Universe from existing data (ongoing discusssion) • 1929 Edwin Hubble ‐ discovery linear redshift‐distance relation (the Hubble relation) ‐ ie. the discovery of the EXPANDING UNIVERSE • 1933 Edward Milne ‐ formulation of the Cosmological Principle ‐ Universe is Isotropic and Homogeneous (on scales larger than 100 million lightyears) • 1933 Fritz Zwicky ‐ discovery of existence of dark matter, from galaxy velocities in Coma cluster of galaxies • 1934 Georges Lemaitre ‐ Cosmological constant (free factor in Einstein field equations): interpretation in terms of vacuum energy with an unusual perfect equation of state • 1946 Evgeni M. Lifschitz ‐ formulation, in a relativistic context, of gravitational instability in an expanding universe, the prevailing theory for the formation of structure in the Universe • 1946 George Gamow ‐ Hot Big Bang ‐ predicts the existence of a cosmic radiation field with a temperature of 50 K (is 2.725K), presuming all chemical elements were formed in the hot Big Bang. • 1948 Ralph Alpher , Hans Bethe, George Gamow ‐ the a‐b‐g paper ‐ describes how the Big bang would by means of nuclear synthesis in the early universe create hydrogen, helium and heavier elements • 1948 Ralph Alpher & Robert Herman ‐ as a consequence of their studies of nucleosynthesis in the early expanding Big Bang universe, theoretical prediction of the existence of a residual, homogeneous, isotropic blackbody radiation ‐ they estimate "the temperature in the universe" at 5K. ‐ in 1965 discovered as the Cosmic Microwave Background Radiation • 1948 Hermann Bondi, Thomas Gold, Fred Hoyle ‐ proposal Steady State Cosmology, based on the perfect cosmological principle • 1950 Fred Hoyle ‐ coins the term Big Bang, meant in a derisive way • 1957 Margaret Burbidge, Geoffrey Burbidge, William Fowler & Fred Hoyle ‐ landmark B2FH paper ‐ Synthesis of the Elements in Stars ‐ describes how all elements, heaver than lithium, are synthesize by nuclear processes in the cores of stars ‐ We are stardust ! Edwin Hubble (1889‐1953) v = H r Hubble Expansion • 1963 Maarten Schmidt ‐ discovery of the first quasar, active nuclei of galaxies visible out to very high redshifts in the Universe • 1965 Arno Penzias & Robert Wilson ‐ Discovery of the 2.7K Microwave Background Radiation (CMB) ‐ ultimate proof of the Hot Big Bang ‐ Nobelprize Physics in 1978 • 1965 Robert Dicke, Jim Peebles, Peter Roll & David Wilkinson ‐ interpretation of the CMB as the relic radiation from the Big Bang • 1966 Jim Peebles ‐ predicts the correct helium abundance, produced as a result of early Universe Big Bang nucleosynthesis • 1966 Stephen Hawking & George Ellis ‐ Singularity Theorem ‐ they show that any plausible general relativistic cosmology is singular 1965: Penzias & Wilson discovery Cosmic Microwave Background Radiation Echo of the Big Bang • 1970 Yakov Zeldovich ‐ Zeldovich formalism ‐ theory of anisotropic gravitational collapse for the formation of structure in the Universe • 1980 Alan Guth, Alexei Starobinsky ‐ Inflationary Big Bang universe ‐ possible solution to the socalled horizon and flatness problems of standard Big Bang models ‐ has become a key element of the standard Big Bang model • 1982‐1984 Jim Peebles, Dick Bond, George Blumenthal ‐ universe dominated by Cold Dark Matter • 1983‐1987 Klypin & Shandarin 1983 Davis, Efstathiou, Frenk & White 1985‐1987 ‐ the first large computer simulations of cosmic structure formation ‐ DEFW show that cold dark matter based simulations produce a reasonable match to observations • 1986 de Lapparent, Geller & Huchra discovery of the Cosmic Web by the CfA2 survey “Slice of the Universe” ‐ final confirmation of earlier suggestions/indictions of a weblike/cellular structure in the Universe ‐ by Einasto et al. (1980) while ‐ later the reality of the Cosmic Web got confirmed in an unambiguous fashion by the maps of the 2dFGRS redshift survey (1997‐2002) • 1990 George Efstathiou, Steve Maddox & Will Sutherland ‐ APM survey: computer processed measurement of the galaxy distribution on the southern sky ‐ first direct detection and claim of the impact of a Cosmological Constant • 1990 COBE CMB satellite, John Mather ‐ precise measurement of the blackbody spectrum of the Cosmic Microwave Background ‐ confirmation of blackbody nature of CMB, to a precision of 1 in 105, the strongest